<?php
if(!$post->post_parent){
	// will display the subpages of this top level page
	$children = wp_list_pages("title_li=&child_of=".$post->ID."&echo=0");
}else{
	// diplays only the subpages of parent level
	//$children = wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0");
	
	if($post->ancestors)
	{
		// now you can get the the top ID of this page
		// wp is putting the ids DESC, thats why the top level ID is the last one
		$ancestors = end($post->ancestors);
		$children = wp_list_pages("title_li=&child_of=".$ancestors."&echo=0");
		// you will always get the whole subpages list
	}
}
if ($children) { ?>
	<ul>
		<?php
		$parent_link = get_permalink($post->post_parent);
		$parent_title = get_the_title($post->post_parent);
		if ($post->post_parent) {
			echo "<li><a href='".$parent_link."'>".$parent_title."</a></li>";
		} else {
			echo "<li class='current_page_item'><a href='".$parent_link."'>".$parent_title."</a></li>";
		}
		?>
		<?php echo $children; ?>
	</ul>
<?php } ?>

Shows the sub pages of a parent page, and also the parent page as a link.

WordPress sub page and parent page menu

php, wordpress

PHP